OSPF Default Route Cost

set on ABR:
area [area-number] default-cost [cost]

OSPF Forwarding Address and How to Suppress It

If the forwarding address in an external LSA is specified, and this address is not reachable, the address contained in the LSA is not inserted into the route table. When NSSA ABR translates the type 7 NSSA LSA into they 5 LSA, by default the forwarding address is transferred from type 7 to type 5. The ABR can be configured to suppress the forwarding address during the translation, replacing the specified address with the address 0.0.0.0. When another router receives the type 5 external LSA with the forwarding address suppressed, instead of trying to direct traffic for the external address to the forwarding address the receiving router will attempt to direct the traffic to the toe 7 to type 5 translating ABR router.

area 10 nssa translate type 7 suppress-fa

Routing TCP/IP Volume 2, Second Edition

area range Command

area range command specifies the area to which the summary adress belongs, the summary address, and the address mask.

It also installs a route to the null interface automatically. The router can be configured to not install it in the route table using the command no discard-route.

no-advertise switch is used for filtering summary addresses to be advertise to backbone network.

Routing TCP/IP Volume 2, Second Edition

Which ABR Translates Type 7 to Type 5

In scenarios which there are more than one ABR connected to the NSSA area, only the ABR with the highest router-id may translate Type 7 LSA(s) to Type 5 LSA(s).

RFC 1587 - 3.4

http://gns3vault.com/OSPF/ospf-nssa-lsa-type-7-to-5-translator-election.html

OSPF External Type 1 and Type 2 Comparison

E1 Routes: Both the external cost and internal OSPF cost matters.
E2 Routes: Only the external cost matters (unless there is a tie).

Use E2 if the goal is to always send traffic through one ASBR.
Use E1 if the goal is to balance the traffic, and make each router pick the closes ASBR.

CCNP-TSHOOT 642-902 Official Certification Guide

Calculating the Cost of Type 2 External Routes-Inter-Area

1. Calculate the cost to rech the ABR, based on the local area's topology.
2. Add the cost from the ABR to the ASBR, as listed in a Type 4 LSA.

CCNP-ROUTE 642-902 Official Certification Guide

Calculating the Cost of Type 2 External Routes-Intra-Area

1. Find the advertising ASBR(s) as listed in the Type 5 LSA(s).
2. Calculate the lowest cost route to reach any of the ASBR(s) based on the area topology.

CCNP-ROUTE 642-902 Official Certification Guide

Calculating the Cost of Inter-Area Routes

1. Calculate the intra-area cost from the router to the ABR listed in the type 3 LSA.
2. Add the cost value listed in the Type 3 LSA

CCNP-ROUTE 642-902 Official Certification Guide

Calculating the Cost of Intra-Area Routes

1. find all subnets inside the area, based on LSA type 1 and 2.
2. Runs SPF and find possible paths.
3. Calculate the OSPF interface costs for all outgoing interfaces and picking the lowest total cost route for each subnet as the best route.

CCNP-ROUTE 642-902 Official Certification Guide

show ip ospf statistics

Provides information about how frequently a router is executing the SFP algorithm.This command also shows when the SPF algorithm last ran and is recommended as the first troubleshooting step for link-state advertisement (LSA) flapping.

Record of reasons causing SPF to be executed:

• N—A change in a network LSA (type 2) has occurred.
• R—A change in a router LSA (type 1) has occurred.
• SA—A change in a Summary autonomous system boundary router (ASBR) (SA) LSA has occurred.
• SN—A change in a Summary Network (SN) LSA has occurred.
• X—A change in an External Type-7 (X7) LSA has occurred.

CCNP-TSHOOT 642-832 Official Certification Guide and http://www.cisco.com/en/US/docs/ios/12_3t/ip_route/command/reference/ip2_s4gt.html#wp1167148

4-Byte AS-PATH

The new AS number is 4-bytes and split into two 2-byte values, in X.Y syntax. The support for the 4-byte AS is advertised via BGP capability negotiation. In order to ensure interoperability with existing BGP peers that do not support 4-byte AS, encoding of BGP OPEN message is reserved and 4-byte AS support is exchanged between the BGP peers via the capability field.



When BGP attempts to establish a session with its peer, the OPEN message may include an optional parameter, called Capabilities. A NEW speaker will include the NEW (4-byte AS) capability when it attempts to OPEN a session with its peer. An OLD speaker should simply ignore the NEW capability advertised by its peer and continue to operate in OLD mode, as detailed in RFC 3392.

If the NEW speaker advertises and receives the 4-byte AS capability from its peer, it will just encode the 4-byte AS number in its AS_PATH or AGGREGATOR attributes when exchanging information with this peer.


If the NEW speaker does not receive the 4-byte AS capability from a particular peer, it indicates this peer is an OLD speaker. Two new attributes are introduced, namely AS4_PATH and AS4_AGGREGATOR. Both attributes are optional transitive. These new attributes use the same encoding as the original ASPATH and AGGREGATOR except the AS Number used is 4-bytes instead of 2-bytes. The NEW speaker will substitute a reserved 2-byte AS number (called AS_TRANS with AS # 23456) for each 4-byte AS so that ASPATH and AGGREGATOR is still 2-byte in length and ASPATH length is still preserved, and at the same time insert the new AS4_PATH and AS4_AGGREGATOR, which will contain the 4-byte encoded copy of the attributes. The NEW speaker will then advertise ASPATH and/or AGGREGATOR together with the AS4_PATH and/or AS4_AGGREGATOR. The OLD speaker that receives these new attributes will preserve and blindly pass them along even though it does not understand them. Subsequent NEW speakers will merge the ASPATH and/or AGGREGATOR with the AS4_PATH and/or AS4_AGGREGATOR to retrieve the original 4-byte AS information without losing any attribute contents, as illustrated in the Figure 1.


http://www.cisco.com/web/about/security/intelligence/4byte-as.html

Troubleshooting Mismatched Duplex

On the full-duplex side:

• High Rcv-Err
• Runts
• FCS-Err

On the half-duplex side:

• High Late-Col counter
• excessive collisions

CCNP-TSHOOT 642-832 - Official Certification Guide

OSPF P-bit

When external routing information is imported into an NSSA in a type 7 link-state advertisement (LSA), the type 7 LSA has only area flooding scope. To further distribute the external information, type 7 LSAs are translated into type 5 LSAs at the NSSA border. The P-bit in the type 7 LSA Options field indicates whether the type 7 LSA should be translated. Only those LSAs with the P-bit set are translated. When you redistribute information into the NSSA, the P-bit is automatically set. A possible workaround applies when the Autonomous System Boundary Router (ASBR) is also an Area Border Router (ABR). The NSSA ASBR can then summarise with the not-advertise keyword, which results in not advertising the translated type 7 LSAs.

http://www.cisco.com/en/US/tech/tk365/technologies_q_and_a_item09186a0080094704.shtml

Cisco Proprietary 802.1D Enhancements vs 802.1w

Cisco proprietary 802.1D enhancements (PortFast, UplinkFast and BackboneFast) are all implemented in 802.1w. Only PortFast requires manual configuration in 802.1w.

STP vs RSTP

802.1D: BPDUs originate from the root bridge and are relayed by all switches down the tree every Hello Time.
802.1w: BPDUs are sent out every switch port at Hello Time intervals, regardless of whether BPDUs are received from the root.

CCNP-SWITCH 642-813 Official Certification Guide

Init bit in EIGRP Update Packet

Suppose you have Routers A and B, running along fine, for many hours. Router A reloads, but comes back up before Router B's hold timer has expired. When Router B sees A's hellos, it will assume that A just missed a couple, and everything is fine. But everything isn't fine--A just lost all of its routing information! How can A signal this state, and as B to re-synchronize?

A can send an empty update, with the init bit set. This causes Router B to place A in the "pending" state, and wipe out all the information it's learned from A (unless, of course, graceful restart is configured/etc).

https://supportforums.cisco.com/thread/50827